1. Field of the Invention
The invention relates to an exhaust-gas catalyst carrier, an exhaust-gas catalyst employing the exhaust-gas catalyst carrier, and a method of manufacturing the exhaust-gas catalyst carrier. More specifically, the invention relates to an exhaust-gas catalyst carrier made of a composite of a specific alumina and a zirconia-titania solid solution that abates sulfur (S) poisoning and exhibit high NOx purification performance even after a long use, an exhaust-gas catalyst employing the exhaust-gas catalyst carrier, and a method of manufacturing the exhaust-gas catalyst carrier.
2. Description of the Related Art
Exhaust gas discharged from the internal combustion engine of a motor vehicle or the like contains HC, CO, and NOx. These substances are discharged into the atmosphere after being purified by an exhaust-gas catalyst. A three-way catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is carried on a porous oxide carrier such as, for example, alumina (Al2O3), silica (SiO2), zirconia (ZrO2), or titania (TiO2) is a representative example of a conventional exhaust-gas catalyst that is widely used.
The three-way catalyst oxidizes and purifies HC and CO contained in exhaust gas, and reduces and purifies NOx contained in exhaust gas. The three-way catalyst exerts its highest effect in an exhaust gas of a stoichiometric atmosphere burned in the vicinity of a stoichiometric air-fuel ratio. Especially in recent years, there have been demands for an improvement in fuel consumption, and exhaust-gas catalysts are increasingly exposed to abrupt atmospheric fluctuations based on A/F fluctuations at high temperatures, due to an increase in the number of times of FC at high temperatures or the like. Such abrupt atmospheric fluctuations substantially promote a deterioration of the catalysts.
Further, although the amount of sulfur components contained in gasoline or kerosene has been reduced, it is impossible to completely remove the sulfur components. Although in small amounts, the sulfur components are inevitably contained in fuel. The sulfur components in fuel deteriorate the performance of the exhaust-gas catalysts. Thus, the development of an exhaust-gas catalyst capable of maintaining its catalytic performance under such fluctuating conditions as well is under way.
For example, Japanese Patent Application Publication No. 2002-316049 (JP-A-2002-316049) describes an exhaust-gas catalyst that is composed of catalyst particles produced by depositing ultrafine particles of a catalyst metal on the surfaces of θ-alumina particles through calcination. Further, Japanese Patent Application Publication No. 2004-167354 (JP-A-2004-167354) describes a catalyst carrier obtained by adding phosphor to a composite oxide in which an acid oxide composed of at least one of ZrO2 and TiO2 and Al2O3 are mixed together on a primary particle level, and an NO oxidation catalyst having a noble metal carried on this catalyst carrier. It is described that it is appropriate to set the temperature for calcinating the carrier lower than 900° C. For example, a carrier powder may be produced by calcining a precipitate that contains Al, Zr, Ti and P components, with the ratio of Zr to (Zr+Ti) equal to about 60 molar %, at 800° C. In addition, paragraph 0023 of JP-A-2004-167354 indicates that it is not preferable to set the molar ratio of zirconia to titania above 70:30 (i.e., to set the molar ratio of zirconia to (zirconia+titania) higher than 70 molar %), because doing so would reduce the heat resistance of the carrier, which would decrease the durability of the carrier.
In addition, Japanese Patent Application Publication No. 2007-229715 (JP-A-2007-229715) describes a catalyst carrier that is composed of an Al2O3—ZrO2—TiO2-type composite oxide with at least a portion of the ZrO2 and TiO2 form a ZrO2—TiO2 solid solution, and a catalyst that includes a noble metal and an NOx occluding material carried on this catalyst carrier. It is described that it is appropriate to set the temperature for calcining the carrier lower than 900° C. An example is described in which a carrier powder is prepared by calcining a precipitate that contains Al, Zr, and Ti components with the ratio of Zr to (Zr+Ti) equal to 70 molar % at 800° C.
However, these exhaust-gas catalysts do not exhibit sufficient catalytic performance after a long use. Therefore, there have been demands for an exhaust-gas catalyst carrier that can exhibit improved catalytic performance, and an exhaust-gas catalyst employing such an exhaust-gas catalyst carrier.